The present invention relates in general to vehicles with electric propulsion, and, more specifically, to monitoring of battery contactor switches by a microcontroller that is digitally isolated from the battery circuits.
The DC power source (e.g., a battery) and other elements of electric drives for hybrid or electric vehicles require monitoring in order to maximize efficiency and performance as well as to detect potential malfunctions. Common battery types such as lithium ion (Li-Ion) use a large number of cells stacked together into a battery pack. Besides monitoring the total voltage output by a battery pack, each cell is typically monitored individually to determine their voltage production and other parameters. The temperature of each cell is also monitored in order to protect against overheating.
It is very challenging to reliably monitor the various battery conditions because of the high-voltage levels involved, the range of intermediate voltages at which respective cells operate within the stack, and the high levels of accuracy required. Various battery monitoring integrated circuit devices have been developed commercially for use in the vehicle environment. Examples of a commercially available battery monitoring IC device include the AD7280A device available from Analog Devices, Inc., of Norwood, Mass., the LTC6804 devices available from Linear Technology Corporation of Milpitas, Calif., and the ISL94212 Multi-Cell Li-Ion Battery Manager available from Intersil Corporation of Milpitas, Calif. In addition to a plurality of inputs for directly monitoring respective battery cells, the known IC devices include several analog inputs for measuring the outputs of thermistor circuits used as temperature sensors for the respective battery cells.
The measured parameters of the battery cells, battery pack, and associated devices are all used by a main microcontroller or microprocessor for performing battery management and communication. Some of the primary components to be monitored include the battery contactor switches that couple the battery pack to the vehicle loads (e.g., the inverter for driving an electric motor). The main micro is typically located in a discrete battery control module or box that interfaces with other vehicle components such as a vehicle system/powertrain controller or a driver interface module. Consequently, the main micro uses a chassis ground for its voltage reference. The chassis ground is isolated from the main battery's reference which is provided at a negative battery bus.
The battery monitoring ICs deployed with the battery pack must monitor the battery cells while being referenced to the negative bus. Therefore, the battery monitoring ICs and any other monitoring devices connected in the high-voltage domain must communicate with the main micro through domain-crossing elements that provide digital isolation between the high-voltage battery domain and the chassis ground domain (i.e., low voltage domain) of the main micro. In order for the main micro to both control the monitoring elements and receive the resulting measured data, relatively expensive components such as photoMOS transistors and a dedicated, high-voltage-referenced analog-to-digital (A/D) converter integrated circuit are typically employed. It would be very desirable to avoid the use of the expensive add-on components while maintaining robust detection of the state of the contactor switches.